Pressure sensors of the above described kind are used in numerous applications, e.g. in connection with operation of combustion engines, chemical process plants and refrigeration systems.
In semiconductor based electronic pressure sensors, a silicon chip with a semiconductor membrane area constitutes the pressure-sensing element. An extensometer is mounted to the membrane area for registering deflection in the membrane area when it is exposed to the pressure of a medium.
In the heretofore known pressure sensors, the silicon chip is typically mounted on a base made of glass. The chip arrangement, constituted by the silicon chip and the glass base, is mounted in a housing comprising a membrane for separation of the silicon chip from the medium. The separating membrane and housing form a sealed space surrounding the chip arrangement. This space is normally filled completely with a pressure-transmitting medium, e.g. silicon oil through which pressure can propagate. In order to transmit the pressure substantially without pressure-loss, the pressure-transmitting medium inside the housing must have a low compressibility. When the separating membrane is exposed to a pressure, the pressure will be transmitted to the silicon chip and cause a deflection of the semiconductor membrane, which deflection is detected by the extensometer.
The required tolerances of the housing are narrow and, accordingly, expensive and time-consuming manufacturing processes have traditionally been used in making pressure sensors of the above-mentioned kind. Typically, the housing, and in particular the cavity therein, is made in a time consuming milling process wherein material is removed from a solid piece of a steel block, or the housing is made by sintering with subsequent machining. Inevitably, this occupies expensive production facilities for a relatively long time.
Since the pressure-transmitting medium normally changes volume differently from the housing when exposed to a change in temperature, the pressure signal deriving from the semiconductor arrangement depends on the temperature. i.e. in response to a specific change in temperature, the volume of the chamber, and the volume of the pressure-transmitting medium changes differently. Since the chamber is completely filled with an incompressible medium, and since the chamber is sealed, the volumetric differences must be compensated by deflection of the flexible membrane thereby introducing a deviation in the pressure signal which deviation depends on the flexibility of the membrane and the temperature. Normally, the semiconductor arrangement is electrically connected to an electronic circuit comprising an arrangement for compensating the temperature caused deviation. Such compensating arrangement is, however, expensive and requires space, and the dimension thereof depends on the necessitated degree of compensation.
In order to reduce the deviation caused by a changing temperature, it is sometimes attempted to limit the amount of free space around the semiconductor in the cavity. The limited space reduces the amount of pressure-transmitting medium necessary to fill up the cavity and thus reduces the impact of changes in the temperature. Both U.S. Pat. Nos. 5,436,491 and 4,502,335 disclose pressure sensors of this kind. These kinds of refined sensors comprise housings made from moulded or machined body casings and further comprise moulded fillers to reduce the free space in the cavity. The design and corresponding manufacturing costs of the sensors are even higher than the costs of making the aforementioned traditional sensors, and the use of fillers introduce additional manufacturing steps.